Isocyanates containing alkoxysilane groups are usable in a versatile manner as heterofunctional units and may find use, for example, in coatings, sealants, adhesives and elastomer materials, but are not limited to these fields of use.
Processes for preparing isocyanates containing alkoxysilane groups are known. For example, they can be obtained by reacting alkoxysilanoalkylamines with phosgene in the presence of tertiary amines (DE 35 44 601 C2, U.S. Pat. No. 9,309,271 B2), although not only the toxicity of phosgene but also the formation of chlorinated by-products and salts is disadvantageous.
Alternatively, access to isocyanates containing alkoxysilane groups can also be achieved via hydrosilylation of isocyanates containing olefin groups in the presence of precious metal catalysts (EP 0 709 392 B1). Disadvantages here are generally inadequate selectivity and high catalyst demand.
A further route to alkoxysilane-containing isocyanates leads via the reaction of haloalkylalkoxysilanes with metal cyanates to form alkoxysilanoalkylurethanes and subsequent thermal cleavage of the urethanes to release the corresponding isocyanates (U.S. Pat. Nos. 3,821,218 A, 3,598,852 A, DE 35 24 215 A1). Disadvantages here are the formation of large amounts of salt and the need to use a solvent, which is typically dimethylformamide.
U.S. Pat. No. 5,218,133 A describes a route to preparation of alkoxysilanoalkylurethanes that avoids the troublesome formation of stoichiometric amounts of salt. For this purpose, alkoxysilanoalkylamines are reacted with alkyl carbonates in the presence of basic catalysts, especially in the presence of metal alkoxides, and the reaction mixture is then neutralized.
Alternatively, the deactivation of the basic catalyst in the reaction mixture can also be conducted with halogenated neutralizing agents (WO 2007/037817 A2). However, these have the disadvantage of leading, in the cleavage reaction described hereinafter, to highly corrosive halogenated substances and neutralization products that make very high demands on the reactor materials and hence increase capital costs and maintenance costs.
U.S. Pat. No. 5,393,910 A describes a process for thermal cracking of alkoxysilanoalkylurethanes prepared preferably according to U.S. Pat. No. 5,218,133 A at high temperature in the gas phase. A disadvantage of this process is the need for special equipment which is stable to high temperature and thus costly. Moreover, patents that do not relate specifically to silanoisocyanates report that the high temperature required leads to reactor carbonization. This is disadvantageous because it is detrimental to plant availability.
As an alternative to urethane cleavage in the gas phase, the thermally induced release of isocyanates containing alkoxysilane groups can also be conducted in a dilute manner in inert solvents (see U.S. Pat. Nos. 5,886,205 A, 6,008,396 A). This involves adding the alkoxysilanoalkylurethane to the inert solvent and choosing a sufficiently high temperature for the solvent as to promote urethane cleavage on the one hand but to avoid unwanted side reactions as far as possible on the other hand. U.S. Pat. No. 5,886,205 A discloses, for the reaction performable in a batchwise or continuous manner, pH values of less than 8, temperatures of not more than 350° C. and a catalyst comprising at least one metal selected from Sn, Sb, Fe, Co, Ni, Cu, Cr, Ti and Pb or at least one metal compound comprising these metals. Disadvantages are the expenditure required for solvent cleaning by comparison with gas phase cleavage, and the unavoidable loss of solvent.
U.S. Pat. No. 9,663,539 B2 describes a process for preparing and subsequently thermally cleaving alkoxysilanoalkylurethanes with the aim of obtaining light-colored isocyanates containing alkoxysilane groups with high storage stability. What is disclosed is a process for preparing isocyanatoorganosilanes, in which    a) an aminoorganosilane is reacted with an organic carbonate ester in the presence of a basic catalyst to give a silyl organocarbamate,    b) the pH of the mixture is adjusted to a pH of not less than 6.0 with an organic carboxylic acid,    c) the mixture obtained is stripped at a temperature of 80-130° C. in order to remove alcohol formed and in order to establish a carbonate ester content of less than about 5.0% by weight,    d) filtration of the mixture from c),    e) optional addition of an organic carboxylic acid in order to adjust the pH to not less than 6.0,    f) thermal cleavage of the mixture obtained in d) or e) in order to obtain an isocyanatoorganosilane and corresponding by-products,    g) separation of the isocyanatoorganosilanes from the by-products obtained in f) and    h) collection of the isocyanatoorganosilanes obtained in g).
Steps a) to c) can be each conducted batchwise or continuously. For step f) in particular, however, no continuous process regime is disclosed. A particular disadvantage of the process described is the lack of selectivity and hence inefficient raw material exploitation.